The present invention relates to a laminated glass and a glass plate used for producing the laminated glass.
Heretofore, tempered glass has mainly been used, in a form of single plate, for a shielding window of automobiles. In recent years, however, laminated glass has been used because of its having good security performance and sound insulating properties. The laminated glass is prepared by interposing an interlayer made of polyvinylbutyral or the like between two glass plates and bonding them. Accordingly, even if crushing to the laminated glass is attempted by knocking it with a blunt instrument, the laminated glass can not easily be penetrated because the interlayer has stretching properties. A criminal actor or actors of car such as robbery or burglar often break a window glass, unlock the door and invade into the cabin. If the invasion is not successful in a short time, he or they may give up the criminal act. Accordingly, it will be effective to use the laminated glass, which is difficult to break, as a window glass from the viewpoint of security for automobile.
There are the following publications in which a laminated glass is used for a window glass.
(1) JP-A-4-231361 (hereinafter xe2x80x9cJP ""361 publicationxe2x80x9d)
The JP ""361 publication discloses a glass plate in which the residual stress in a peripheral region (region A1) of the glass plate and the residual stress in a central region (region C1) of the glass plate are described.
(2) JP-A-6-503063 (hereinafter xe2x80x9cJP ""063 publicationxe2x80x9d)
The JP ""063 publication discloses a glass plate in which the residual stress in a peripheral region (region A2), the residual stress in a central region (region C2) and an intermediate region (region B2) between the region A2 and the region C2 are described.
(3) JP-A-6-87328 (hereinafter xe2x80x9cJP ""338 publicationxe2x80x9d)
The JP ""328 publication discloses a glass plate in which the residual stress in an edge region (region B3) adjacent to a peripheral region (region A3) and the residual stress in a central region (region C3) are described.
In each of the publications, the distribution of stress values of a glass plate is described specifically using concrete numerical values which are shown in Tables 1 to 3. In the tables, each tensile stress in a core portion (or an inner portion) indicates the principal stress difference at an intermediate point in a thickness direction of the glass plate, which is known in principle xc2xd of the compression stress in the surface portion. Accordingly, in Tables 1 and 2 described below, the compression stress in the surface portion which can be estimated from the tensile stress in the core portion is also described. Further, the stress in the edge portion is the principal stress difference in a peripheral portion of the glass plate. In this case, a positive value indicates a compression stress and a negative value indicates a tensile stress.
For example, when two principal stresses perpendicularly crossing to each other are both compression stresses, the principal stress difference has compressive properties, and when two principal stresses perpendicularly crossing to each other are both tensile stresses, the principal stress difference has tensile properties. When either of two principal stresses is a compression stress and the other is a tensile stress, the principal stress difference has compressive properties when the principal stress of compression is stronger, and has tensile properties when the principal stress of tensile is stronger.
Here, the generally used method for producing tempered glass and the mechanism of generating the residual stress will be described. The tempered glass is produced by forming a residual compression stress layer in the surface of a glass plate and at the same time, forming a residual tensile stress layer in the core portion. Specifically, the residual stress layers are formed by causing a temperature difference between the surface and the core portion of the glass plate by blowing cooling air to the surface of the glass plate heated to nearly the softening point. If the glass plate has a certain thickness and infinite surface dimensions and if both surfaces are cooled uniformly, the distribution of the stress along its thickness direction exhibits a distribution of substantially parabolic shape. Further, the compression stress in the surface becomes twice as large as the tensile stress in the center in a thickness direction of the glass plate, and the integrated value of stress along the thickness direction becomes xe2x80x9c0xe2x80x9d.
However, glass plates have actually finite dimensions, and end surfaces exist in the peripheral portion. Accordingly, the glass plate is cooled from not only the surface but also the end surfaces, whereby a region (peripheral region) having a width two or three times as much as the thickness of the glass plate in the peripheral portion in which the principal stress difference averaged in the thickness direction has compressive properties, is produced. Further, at an inner peripheral side of the peripheral region, there is formed a region (intermediate region) in which the principal stress difference averaged in the thickness direction has tensile properties, so as to balance with the compression stress in the peripheral region.
Next, description will be made as to the principal stress difference. When a plane perpendicular to a main surface of the glass plate (a plane cross-sectioned perpendicularly to a main surface of the glass plate) is selected, such plane can take any angle with respect to a linear line extending parallel to the main surface of the glass plate. When a single point is selected in such plane, the stress value acting on this point and having a direction perpendicular to the selected plane can take different values depending on an angle of the selected plane. In various angles, there are an angle at which the stress value is maximal and an angle at which the stress value is minimal. The principal stress direction includes a direction of stress indicating the maximum value and a direction of stress indicating the minimum value, which is perpendicular to the direction indicating the maximum value. In this specification, the direction of stress indicating the maximum value is called as the principal stress direction.
However, the principal stress itself can not directly be measured. Therefore, the principal stress is evaluated indirectly by using the principal stress difference obtained by a photoelasticity method. The principal stress difference obtained by the photoelasticity method corresponds to a value obtained by dividing the sum of the values which are obtained by subtracting the minimum value of stress from the maximum value of stress at each point in all points arranged in a thickness direction of the plate, by the thickness of the plate. Namely, the principal stress difference corresponds to an averaged value obtained by dividing an integrated value of the maximum value of stress minus the minimum value of stress by the thickness of the plate at each point. Accordingly, when a certain point on a main surface of the glass plate is chosen, an average of the integrated value obtained by subtracting the minimum value of stress from the maximum value of stress at each point in the thickness direction of the plate from the selected point is the principal stress difference at the selected point. Further, the principal stress direction in this case is the principal stress direction at the selected point. Further, principal stress direction in this specification is a direction parallel to the main surface of the glass plate.
As is clear from the numerical values described in the last right column in Tables 1 to 3, any averaged compression stress of the surface (hereinbelow, referred to as the averaged surface compression stress) in the central region is 35 MPa or more which is sufficient in strength against an impact force. However, the glass plate having a strength of 35 MPa or more is hardly broken when a human body hits the glass plate in car crushing or the like, and the glass plate may not absorb sufficiently the impact force whereby the human body may be damaged. Accordingly, it is necessary to reduce the averaged surface compression stress in the central region so that the glass plate can be broken.
On the other hand, a door glass of an automobile can be slided vertically so as to open and close the window, and an upper side portion of the glass plate is exposed in an open state of window. Since the exposed upper side portion is apt to receive an impact due to the contact of a passenger or an impact at the time of opening or closing the glass plate, it should have a strength sufficient to withstand such external force. Further, a side edge portion of the sliding window is fixed to the window sash, and a lower edge portion thereof is a portion used for attaching the regulator as a power source for sliding the glass plate. Accordingly, these portions are apt to receive a mechanical external force. Thus, since various external forces are applied to the peripheral portion of the sliding window, the peripheral portion of the sliding window should have a large strength in comparison with the central region.
As understood from the above-mentioned, in the laminated glass used for a sliding window for an automobile, the peripheral portion of it should have a sufficient strength durable to various external forces and the central region having a strength of such extent that when an excessive impact due to the contact of a human body will applied, it can be broken. In the above-mentioned publications, however, there is no disclosure of a glass plate and a laminated glass satisfying these conditions.
It is an object of the present invention to eliminate the above-mentioned problems in the conventional techniques and to provide a laminated glass suitable for a sliding window of an automobile or the like and a glass plate used for producing the laminated glass.
In accordance with the present invention, there is provided a glass plate characterized in that the thickness of the plate is from 1.5 to 3 mm, and it has a peripheral region having a predetermined width along the edge line of the plate and having its principal stress difference of compressive properties, an intermediate region having a predetermined width adjacent to an inner periphery of the peripheral region and having its principal stress difference of tensile properties and a central region occupying an inner peripheral side of the intermediate region wherein the central region has an averaged surface compression stress of from 15 to 35 MPa, and the peripheral region has a width of from 5 to 20 mm; the maximum value of the principal stress difference averaged in a thickness direction of the plate along its center line is from 20 to 40 MPa, and the minimum value of the principal stress difference averaged in a thickness direction along the center line is from 8 to 25 MPa.
Further, in accordance with the present invention, there is provided a laminated glass comprising two or more glass plates and an interlayer bonded between adjacent glass plates wherein at least one of the glass plates is such one as above-mentioned.